1. Field of the Invention
The present invention relates to the field of optical systems, and more particularly, to the field of lens mounts which compensate for changes in the properties of optical systems as the temperature of the system changes.
2. Background Art
It is frequently desirable to have a lens mounted so that the image it transmits can be focused on a detector over a wide range of temperatures. However, the indices of refraction of all lens materials exhibit some variation with temperature, and as a result, the focusing properties of an optical system including the lens are altered. Lens designs and manufacturing processes have been developed to minimize this effect, but for many applications the residual temperature dependence is sufficient to pose problems.
Changes in the focal length of a lens are of little consequence where the lens is part of an optical system that focuses visible light. In these cases, for example, with a pair of binoculars, a user monitors the image and adjusts the optical system to compensate for any changes in focal length of the lens with temperature. The situation is very different where the lens focuses an image onto a focal plane array or detector, which subsequently processes the focused light rays. Such arrangements are common, for example, in military applications involving radiation in the infrared (IR) wavelength region. In this situation, no human user can monitor and readjust the focus of the optical system in response to thermally induced changes in the index of refraction of the lens material. Consequently, it is desirable that these optical systems include some built-in means to compensate for thermally induced changes in the optical system.
In many types of lenses, particularly those used for IR radiation, the index of refraction of the lens material changes in a manner that decreases the focal length of the lens as the temperature increases. On the other hand, lens mounts commonly used to support a lens above its focal plane are constructed of materials having positive coefficients of thermal expansion (CTE). The expansion of these material with rising temperatures thus increases the distance between the lens and the focal plane. This change reinforces the defocusing effects attributable to the decrease in the focal length of the lens with increasing temperature.
Previous attempts to deal with this problem have employed a reentrant tube geometry for the lens mount. Reentrant tube lens mounts comprise three precision-machined mounting tubes having different thermal properties. The mounting tubes are arranged in a coaxial geometry and connected so that the net CTE of the three tube structure moves the lens in the desired direction as the temperature varies. This mechanically complex structure makes it difficult to construct reentrant tube lens mounts that are sufficiently rugged to withstand use in high vibration environments such as ground vehicles or aircraft, especially rotary wing air craft. Further, it is sometimes necessary to provide a special atmosphere such as dry air or nitrogen within the lens mount structure. The complex structure of reentrant tube lens mounts makes it difficult to establish and maintain the seals necessary to preserve the special atmosphere. In addition, it is difficult to find materials that have sufficiently high CTEs for the degree of compensating motion that may be necessary. In some cases, plastic materials are used for the intermediate tube. However, little is known about the long-term repeatability of the CTE behavior of these materials. For these reasons, there is a need for a thermally compensated lens mount that requires neither complex geometries nor unusual or untested material.